Method of fabricating transparent contacts for organic devices

ABSTRACT

A multicolor organic light emitting device employs vertically stacked layers of double heterostructure devices which are fabricated from organic compounds. The vertical stacked structure is formed on a glass base having a transparent coating of ITO or similar metal to provide a substrate. Deposited on the substrate is the vertical stacked arrangement of three double heterostructure devices, each fabricated from a suitable organic material. Stacking is implemented such that the double heterostructure with the longest wavelength is on the top of the stack. This constitutes the device emitting red light on the top with the device having the shortest wavelength, namely, the device emitting blue light, on the bottom of the stack. Located between the red and blue device structures is the green device structure. The devices are configured as stacked to provide a staircase profile whereby each device is separated from the other by a thin transparent conductive contact layer to enable light emanating from each of the devices to pass through the semitransparent contacts and through the lower device structures while further enabling each of the devices to receive a selective bias. The devices are substantially transparent when de-energized, making them useful for heads-up display applications.

This is a Divisional Application of U.S. Ser. No. 08/613,207 filed onMar. 6, 1996, now U.S. Pat. No. 5,703,436 and a continuation-in-part ofU.S. Ser. No. 08/354,674, filed on Dec. 13, 1994, now U.S. Pat. No.5,707,745.

FIELD OF THE INVENTION

This invention relates to multicolor organic light emitting devices, andmore particularly to such devices for use in flat panel electronicdisplays, heads-up displays, and so forth.

BACKGROUND OF THE INVENTION

The electronic display is an indispensable way in modern society todeliver information and is utilized in television sets, computerterminals and in a host of other applications. No other medium offersits speed, versatility and interactivity. Known display technologiesinclude plasma displays, light emitting diodes (LEDs), thin filmelectroluminescent displays, and so forth.

The primary non-emissive technology makes use of the electro opticproperties of a class of organic molecules known as liquid crystals(LCs) or liquid crystal displays (LCDs). LCDs operate fairly reliablybut have relatively low contrast and resolution, and require high powerbacklighting. Active matrix displays employ an array of transistors,each capable of activating a single LC pixel. There is no doubt that thetechnology concerning flat panel displays is of a significant concernand progress is continuously being made. See an article entitled “FlatPanel Displays”, Scientific American, March 1993, pgs. 90-97 by S. W.Depp and W. E. Howard. In that article, it is indicated that by 1995flat panel displays alone are expected to form a market of between 4 and5 billion dollars. Desirable factors for any display technology is theability to provide a high resolution full color display at good lightlevel and at competitive pricing.

Color displays operate with the three primary colors red (R), green (G)and blue (B). There has been considerable progress in demonstrating red,green and blue light emitting devices (LEDs) using organic thin filmmaterials. These thin film materials are deposited under high vacuumconditions. Such techniques have been developed in numerous placesthroughout the world and this technology is being worked on in manyresearch facilities.

Presently, the most favored high efficiency organic emissive structureis referred to as the double heterostructure LED which is shown in FIG.1A and designated as prior art. This structure is very similar toconventional, inorganic LED's using materials as GaAs or InP.

In the device shown in FIG. 1A, a support layer of glass 10 is coated bya thin layer of Indium Tin Oxide (ITO) 11, where layers 10 and 11 formthe substrate. Next, a thin (100-500 Å) organic, predominantly holetransporting layer (HTL) 12 is deposited on the ITO layer 11. Depositedon the surface of HTL layer 12 is a thin (typically, 50 Å-100 Å)emission layer (EL) 13. If the layers are too thin there may be lack ofcontinuity in the film, and thicker films tend to have a high internalresistance requiring higher power operation. Emissive layer (EL) 13provides the recombination site for electrons injected from a 100-500 Åthick electron transporting layer 14 (ETL) with holes from the HTL layer12. The ETL material is characterized by its considerably higherelectron than hole mobility. Examples of prior art ETL, EL and HTLmaterials are disclosed in U.S. Pat. No. 5,294,870 entitled “OrganicElectroluminescent MultiColor Image Display Device”, issued on Mar. 15,1994 to Tang et al.

Often, the EL layer 13 is doped with a highly fluorescent dye to tunecolor and increase the electroluminescent efficiency of the LED. Thedevice as shown in FIG. 1A is completed by depositing metal contacts 15,16 and top electrode 17. Contacts 15 and 16 are typically fabricatedfrom indium or Ti/Pt/Au. Electrode 17 is often a dual layer structureconsisting of an alloy such as Mg/Ag 17′ directly contacting the organicETL layer 14, and a thick, high work function metal layer 17″ such asgold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17″ is opaque.When proper bias voltage is applied between top electrode 17 andcontacts 15 and 16, light emission occurs through the glass substrate10. An LED device of FIG. 1A typically has luminescent external quantumefficiencies of from 0.05 percent to 4 percent depending on the color ofemission and its structure.

Another known organic emissive structure referred as a singleheterostructure is shown in FIG. 1B and designated as prior art. Thedifference in this structure relative to that of FIG. 1A, is that the ELlayer 13 serves also as an ETL layer, eliminating the ETL layer 14 ofFIG. 1A. However, the device of FIG. 1B, for efficient operation, mustincorporate an EL layer 13 having good electron transport capability,otherwise a separate ETL layer 14 must be included as shown for thedevice of FIG. 1A.

Presently, the highest efficiencies have been observed in green LED's.Furthermore, drive voltages of 3 to 10 volts have been achieved. Theseearly and very promising demonstrations have used amorphous or highlypolycrystalline organic layers. These structures undoubtedly limit thecharge carrier mobility across the film which, in turn, limits currentand increases drive voltage. Migration and growth of crystallitesarising from the polycrystalline state is a pronounced failure mode ofsuch devices. Electrode contact degradation is also a pronounced failuremechanism.

Yet another known LED device is shown in FIG. 1C, illustrating a typicalcross sectional view of a single layer (polymer) LED. As shown, thedevice includes a glass support layer 1, coated by a thin ITO layer 3,for forming the base substrate. A thin organic layer 5 of spin-coatedpolymer, for example, is formed over ITO layer 3, and provides all ofthe functions of the HTL, ETL, and EL layers of the previously describeddevices. A metal electrode layer 6 is formed over organic layer 5. Themetal is typically Mg, Ca, or other conventionally used metals.

An example of a multicolor electroluminescent image display deviceemploying organic compounds for light emitting pixels is disclosed inTang et al., U.S. Pat. No. 5,294,870. This patent discloses a pluralityof light emitting pixels which contain an organic medium for emittingblue light in blue-emitting subpixel regions. Fluorescent media arelaterally spaced from the blue-emitting subpixel region. The fluorescentmedia absorb light emitted by the organic medium and emit red and greenlight in different subpixel regions. The use of materials doped withfluorescent dyes to emit green or red on absorption of blue light fromthe blue subpixel region is less efficient than direct formation viagreen or red LED's. The reason is that the efficiency will be theproduct of (quantum efficiency for EL)*(quantum efficiency forfluorescence)*(1-transmittance). Thus a drawback of this display is thatdifferent laterally spaced subpixel regions are required for each coloremitted.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multicolor organiclight emitting device employing several types of organicelectroluminescent media, each for emitting a distinct color.

It is a further object of this invention to provide such a device in ahigh definition multicolor display in which the organic media arearranged in a stacked configuration such that any color can be emittedfrom a common region of the display.

It is another object of the present invention to provide a three colororganic light emitting device which is extremely reliable, substantiallytransparent when de-energized, and relatively inexpensive to produce.

It is a further object to provide such a device which is implemented bythe growth of organic materials similar to those materials used inelectroluminescent diodes, to obtain an organic LED which is highlyreliable, compact, efficient and requires low drive voltages forutilization in RGB displays.

In one embodiment of the invention, a multicolor light emitting device(LED) structure comprises at least a first and a second organic LEDstacked one upon the other, and preferably three, to form a layeredstructure, with each LED separated one from the other by a transparentconductive layer to enable each device to receive a separate biaspotential to emit light through the stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view of a typical organic doubleheterostructure light emitting device (LED) according to the prior art.

FIG. 1B is a cross sectional view of a typical organic singleheterostructure light emitting device (LED) according to the prior art.

FIG. 1C is a cross sectional view of a known single layer polymer LEDstructure according to the prior art.

FIGS. 2A, 2B, and 2C are cross sectional views of an integrated threecolor pixel utilizing crystalline organic light emitting devices(LED's), respectively, according to embodiments of this invention,respectively.

FIGS. 3-11 show a variety of organic compounds which may be used tocomprise the active emission layers for generating the various colors.

FIGS. 12(A-E) illustrate a shadow masking process for the fabrication ofthe multicolor LED according to the invention.

FIGS. 13(A-F) illustrate a dry etching process for the fabrication ofthe multicolor LED according to the invention.

FIG. 14A shows a multicolor LED of one embodiment of this inventionconfigured for facilitating packaging thereof.

FIG. 14B shows a cross sectional view of a hermetic package for anotherembodiment of the invention.

FIG. 14C is cross sectional view taken along 14C—14C of FIG. 14B.

FIG. 15 is a block diagram showing an RGB display utilizing LED devicesaccording to this invention together with display drive circuitry.

FIG. 16 shows an LED device of another embodiment of the inventionextending the number of stacked LED's to N, where N is an integer number1, 2, 3 . . . N.

FIG. 17 shows a substantially transparent organic light emitting device(TOLED) for another embodiment of the invention.

FIG. 18 shows the spectral output of a TOLED emitting from a 100 Å thickMg—Ag contact (dotted line) and the associated substrate (solid line) asa plot of EL Intensity (a.u.) versus Wavelength (nm), for the embodimentof the invention of FIG. 17.

FIG. 19 is a plot of Transmission (%) versus Wavelength (nm) for showingan example of the transmission spectrum of the TOLED device of FIG. 17with a 100 Å thick Mg—Ag electrode as a function of wavelength.

FIG. 20 shows a plot of Transmission (%) vs. Mg—Ag contact thickness ata wavelength of 530 nm, for an engineering prototype of the TOLED topcontact of FIG. 17.

FIG. 21 shows a system for depositing a Mg:Ag film onto an organic layeron a substrate for one embodiment of the invention for partially forminga transparent contact on an organic layer.

FIG. 22 shows a block diagram generally showing the three successiveoperations that must be carried out for forming the transparent contacton an organic layer for one embodiment of the invention.

FIG. 23 shows a sputtering system for depositing an indium-tin oxide(ITO) film on top of a Mg:Ag metal alloy film previously deposited on anorganic layer for one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A has been described and is a prior art double heterostructureorganic light emitting device. The basic construction of the device ofFIG. 1A is used in this invention as will be described.

Referring to FIG. 2A, there is shown a schematic cross section of ahighly compact, integrated RGB pixel structure which is implemented bygrown or vacuum deposited organic layers, in one embodiment of theinvention. Based on the ability to grow organic materials on a largevariety of materials (including metals and ITO), one can construct astack of LED double heterostructures (DH) designated as 20, 21 and 22,in one embodiment of the invention. For illustrative purposes, LED 20 isconsidered in a bottom portion of the stack, LED 21 in a middle portionof the stack, and LED 22 in a top portion of the stack, in the exampleof FIG. 2A. Also, the stack is shown to be vertically oriented in FIG.2A, but the LED can be otherwise oriented. In other embodiments, a stackof single heterostructure (SH) LED's (see FIG. 1B), or a stack ofpolymer-based LED devices (see FIG. 1C), are viable alternatives to theDH LED's, with the SH devices being equally viable as DH devices forlight emitters. Also, SH and DH devices comprising a combination ofvacuum deposited and polymeric light-emitting materials are consideredto be within the spirit and scope of this invention.

Each device structure as device 20, consists of an HTL layer 20Hvacuum-deposited or grown on or otherwise deposited onto the surface ofan ITO layer 35. A top ETL layer 20T sandwiches an EL layer 20E betweenthe former and HTL layer 20H, for example, shown in the deviceconstruction of FIG. 2A. The ETL layer 20T and other ETL layers to bedescribed are composed of organic materials such asM(8-hydroxyquinolate)_(n)(M=metal ion; n=2−4). Examples of othersuitable organic ETL materials can be found in U.S. Pat. No. 5,294,870to Tang et al. Formed on top of ETL layer 20T is a thin,semi-transparent low work function (preferably, <4 eV) metal layer 26Mhaving a thickness typically ranging from 50 Å to 400 Å. Suitablecandidates include Mg, Mg/Ag, and As. Deposited on the top of metallayer 26M is another transparent, thin conductive ITO layer 26I. (Forconvenience herein, the double layer structure of metallic layer 26M andITO layer 26I is referred to as ITO/metal layers 26.) Each of the doubleheterostructure devices as 20, 21 and 22 have a bottom HTL layer formedon a transparent conductive layer of ITO 26I or 35. Next an EL layer isdeposited and then another layer of ETL. Each of the HTL, ETL, ITO,metal and organic EL layers are transparent because of their compositionand minimal thickness. Each HTL layer may be 50 Å to greater than 1000 Åthick; each EL layer may be 50 to greater than 200 Å thick; each ETLlayer may be 50 Å to greater than 1000 Å thick; each metal layer 26M maybe 50 Å to greater than 100 Å thick; and each ITO layer 261 and 35 maybe 1000 Å to greater than 4000 Å thick. For optimum performance, each ofthe layers should preferably be kept towards the lower ends of the aboveranges, but these ranges are not meant to be limiting. Thus, each LED20, 21 and 22 (excluding ITO/metal layers) are preferably close to 200 Åthick.

If SH LED devices are used for providing LED's 20, 21, 22, rather thanDH LED devices, the ETL and EL layers are provided by a single layer,such as layer 13, as previously described for the SH of FIG. 1B. Thislayer 13 is typically Al-quinolate. This is shown in FIG. 2B, where theEL layers 20E, 21E, and 22E, respectively, provide both the EL and ETLlayer functions. However, an advantage of the DH LED stack of FIG. 2A,relative to a SH LED stack of FIG. 2B, is that the DH LED stack permitsthinner overall construction with high efficiency.

In FIGS. 2A and 2B, even though the centers of each of the LED's areoffset from one another, the total beam of light from each device issubstantially coincident between LED's 20, 21 and 22. While the beams oflight are coincident in the concentric configuration, the emitting ornon-emitting device closer to the glass substrate will be transparent tothe emitting device or devices further away from the glass substrate.However, the diodes 20, 21 and 22 need not be offset from one anotherand may alternatively be stacked concentrically upon each other,whereupon the beam of light from each device is wholly coincident withthe others. A concentric configuration is shown in FIG. 12E which willbe described below in regard to device fabrication processes. Note thatthere is no difference in function between the offset and concentricconfigurations. Each device emits light through glass substrate 37 in asubstantially omnidirectional pattern. The voltages across the threeLED's in the stack 29 are controlled to provide a desired resultantemission color and brightness for the particular pixel at any instant oftime. Thus, each LED as 22, 21 and 20 can be energized simultaneouslywith beams as R, G and B, respectively, for example, directed throughand visible via the transparent layers, as shown schematically in FIGS.2A and 2B. Each DH structure 20, 21 and 22 is capable upon applicationof a suitable bias voltage of emitting a different color light. Thedouble heterostructure LED 20 emits blue light. The doubleheterostructure LED 21 emits green light while the doubleheterostructure (DH) LED 22 emits red light., Different combinations orindividual ones of LED's 20, 21 and 22 can be activated to selectivelyobtain a desired color of light for the respective pixel partlydependent upon the magnitude of current in each of the LED's 20, 21 and22.

In the example of FIGS. 2A and 2B, LED's 20, 21 and 22 are forwardbiased by batteries 32, 31 and 30, respectively. Current flows from thepositive terminal of each battery 32, 31 and 30, into the anodeterminals 40, 41, 42, respectively, of its associated LED 20, 21 and 22,respectively, through the layers of each respective device, and fromterminals 41, 42 and 43, serving as cathode terminals to negativeterminals of each battery 32, 31, and 30, respectively. As a result,light is emitted from each of the LED's 20, 21 and 22. The LED devices20, 21 and 22 are made selectively energizable by including means (notshown) for selectively switching batteries 32, 31 and 30, respectively,into and out of connection to their respective LED.

In the embodiments of the invention, relative to FIGS. 2A and 2B, thetop ITO contact 26I for LED 22 is transparent, making the three colordevice shown useful for heads-up display applications. However, inanother embodiment of the invention, the top contact 26I is formed froma thick metal, such as either Mg/Ag, In, Ag, or Au, for reflecting lightemitted upward back through substrate 13, for substantially increasingthe efficiency of the device. Also, overall device efficiency can beincreased by forming a multilayer dielectric thin film coating betweenglass substrate 37 and the ITO layer 35, to provide an anti-reflectingsurface. Three sets of anti-reflecting layers are required, one to forman anti-reflection coating at each wavelength emitted from the variouslayers.

In another embodiment, the device of FIG. 2A is constructed in anopposite or inverted manner, for providing light emission out of the topof stack rather than the bottom as the former. An example of an invertedstructure, with reference to FIG. 2C, is to replace ITO layer 35 with athick, reflective metal layer 38. Blue LED 20 is then provided byinterchanging HTL layer 20H and ETL layer 20T, with EL layer 20Eremaining sandwiched between the latter two layers. Furthermore, themetal contact layer 26M is now deposited on top of ITO layer 26I. Thegreen LED 21 and red LED 22 portions of the stack are each constructedwith inverted layers (the HTL and ETL layers of each are interchanged,followed by inverting the metal and ITO layers) as described for theinverted blue LED 20. Note that in the inverted structure, the bluedevice 20 must be on top and the red device 22 on the bottom. Also, thepolarities of batteries 30, 31, and 32 are reversed. As a result, thecurrent flow through devices 20, 21 and 22, respectively, is in the samedirection relative to the embodiment of FIG. 2A, when forward biased foremitting light.

The device in the cross sectional view has a step-like or staircaseprofile, in this example. The transparent contact areas (ITO) 26I permitseparate biasing of each pixel element in the stack and furthermore thematerial can be used as an etch stop during the processing steps. Theseparate biasing of each DH LED structure 20, 21 and 22 allows forwavelength tuning of the pixel output to any of various desired colorsof the visible spectrum as defined in the CIE (Commission Internationalede l'Eclairage/International Commission of Illumination) chromaticitystandard. The blue emitting LED 20 is placed at the bottom of the stackand it is the largest of the three devices. Blue is on the bottombecause it is transparent to red and green light. Finally, the materials“partitioning” using the transparent ITO/metal layers 26 facilitatesmanufacture of this device as will be described. It is the very uniqueaspects of the vacuum growth and fabrication processes associated withorganic compounds which makes the pixel LED devices shown in FIGS. 2A,2B, and 2C possible. The vertical layering shown in FIGS. 2A, 2B, and 2Callows for the fabrication of three color pixels with the smallestpossible area, hence, making these ideal for high definition displays.

As seen in FIGS. 2A, 2B, and 2C, each device DH structure 20, 21 and 22can emit light designated by arrows B, G and R, respectively, eithersimultaneously or separately. Note that the emitted light is fromsubstantially the entire transverse portion of each LED 20, 21 and 22,whereby the R, G, and B arrows are not representative of the width ofthe actual emitted light, respectively. In this way, the addition orsubtraction of colors as R, G and B is integrated by the eye causingdifferent colors and hues to be perceived. This is well known in thefield of color vision and display colorimetry. In the offsetconfiguration shown, the red, green and blue beams of light aresubstantially coincident. Any one of a variety of colors can be producedfrom the stack, and it will appear as one color originating from asingle pixel.

The organic materials used in the DH structures are grown one on top ofthe other or are vertically stacked with the longest wavelength device22 indicative of red light on the top and the shortest wavelengthelement 20 indicative of blue light on the bottom. In this manner, oneminimizes light absorption in the pixel or in the devices. Each of theDH LED devices are separated by ITO/metal layers 26 (specifically,semitransparent metal layers 26M, and indium tin oxide layers 26I). TheITO layers 26I can further be treated by metal deposition to providedistinct contact areas on the exposed ITO surfaces, such as contacts 40,41, 42 and 43. These contacts 40, 41, 42 and 43 are fabricated fromindium, platinum, gold, silver or alloys such as Ti/Pt/Au, Cr/Au, orMg/Ag, for example. Techniques for deposition of contacts usingconventional metal deposition or vapor deposition are well known. Thecontacts, such as 40, 41, 42 and 43, enable separate biasing of each LEDin the stack. The significant chemical differences between the organicLED materials and the transparent electrodes 26I permits the electrodesto act as etch stop layers. This allows for the selective etching andexposure of each pixel element during device processing.

Each LED 20, 21, 22 has its own source of bias potential, in thisexample shown schematically as batteries 32, 31, and 30, respectively,which enables each LED to emit light. It is understood that suitablesignals can be employed in lieu of the batteries 30, 31, 32,respectively. As is known, the LED requires a minimum threshold voltageto emit light (each DH LED) and hence this activating voltage is shownschematically by the battery symbol.

The EL layers 20E, 21E, 22E may be fabricated from organic compoundsselected according to their ability to produce all primary colors andintermediates thereof. The organic compounds are generally selected fromtrivalent metal quinolate complexes, trivalent metal bridged quinolatecomplexes, Schiff base divalent metal complexes, tin (iv) metalcomplexes, metal acetylacetonate complexes, metal bidentate ligandcomplexes, bisphosphonates, divalent metal maleonitriledithiolatecomplexes, molecular charge transfer complexes, aromatic andheterocyclic polymers and rare earth mixed chelates, as describedhereinafter.

The trivalent metal quinolate complexes are represented by thestructural formula shown in FIG. 3, wherein M is a trivalent metal ionselected from Groups 3-13 of the Periodic Table and the Lanthanides.Al⁺³, Ga⁺³ and In⁺³ are the preferred trivalent metal ions.

R of FIG. 3 includes hydrogen, substituted and unsubstituted alkyl, aryland heterocyclic groups. The alkyl group may be straight or branchedchain and preferably has from 1 to 8 carbon atoms. Examples of suitablealkyl groups are methyl and ethyl. The preferred aryl group is phenyland examples of the heterocyclic group for R include pyridyl, imidazole,furan and thiophene.

The alkyl, aryl and heterocyclic groups of R may be substituted with atleast one substituent selected from aryl, halogen, cyano and alkoxy,preferably having from 1 to 8 carbon atoms. The preferred halogen ischloro.

The group L of FIG. 3 represents a ligand including picolylmethylketone,substituted and unsubstituted salicylaldehyde (e.g. salicylaldehydesubstituted with barbituric acid), a group of the formula R(O)CO—wherein R is as defined above, halogen, a group of the formula RO—wherein R is as defined above, and quinolates (e.g. 8-hydroxyquinoline)and derivatives thereof (e.g. barbituric acid substituted quinolates).Preferred complexes covered by the formula shown in FIG. 3 are thosewhere M is Ga⁺³ and L is chloro. Such compounds generate a blueemission. When M is Ga⁺³ and L is methyl carboxylate, complexes emittingin the blue to blue/green region are produced. A yellow or red emissionis expected by using either a barbituric acid substitutedsalicylaldehyde or a barbituric acid substituted 8-hydroxyguinoline forthe L group. Green emissions may be produced by using a quinolate forthe L group.

The trivalent metal bridged quinolate complexes which may be employed inthe present invention are shown in FIGS. 4A and 4B. These complexesgenerate green emissions and exhibit superior environmental stabilitycompared to trisquinolates (complexes of FIG. 3 where L is a quinolate)used in prior art devices. The trivalent metal ion M used in thesecomplexes is as defined above with Al⁺³, Ga⁺³, or In⁺³ being preferred.The group Z shown in FIG. 4A has the formula SiR wherein R is as definedabove. Z may also be a group of the formula P═O which forms a phosphate.

The Schiff base divalent metal complexes include those shown in FIGS. 5Aand 5B wherein M¹ is a divalent metal chosen from Groups 2-12 of thePeriodic Table, preferably Zn (See, Y. Hanada, et al., “BlueElectroluminescence in Thin Films of Axomethin—Zinc Complexes”, JapaneseJournal of Applied Physics Vol. 32, pp. L511-L513 (1993). The group R′is selected from the structural formulas shown in FIGS. 5A and 5B. TheR¹ group is preferably coordinated to the metal of the complex throughthe amine or nitrogen of the pyridyl group. X is selected from hydrogen,alkyl, alkoxy, each having from 1 to 8 carbon atoms, aryl, aheterocyclic group, phosphino, halide and amine. The preferred arylgroup is phenyl and the preferred heterocyclic group is selected frompyridyl, imidazole, furan and thiophene. The X groups affect thesolubility of the Schiff base divalent metal complexes in organicsolvents. The particular Schiff base divalent metal complex shown inFIG. 5B emits at a wavelength of 520 nm.

The tin (iv) metal complexes employed in the present invention in the ELlayers generate green emissions. Included among these complexes arethose having the formula SnL¹ ₂L² ₂ where L¹ is selected fromsalicylaldehydes, salicylic acid or quinolates (e.g.8-hydroxyquinoline). L² includes all groups as previously defined for Rexcept hydrogen. For example, tin (iv) metal complexes where L¹ is aquinolate and L² is phenyl have an emission wavelength (λ_(cm)) of 504nm, the wavelength resulting from measurements of photoluminescence inthe solid state.

The tin (iv) metal complexes also include those having the structuralformula of FIG. 6 wherein Y is sulfur or NR² where R² is selected fromhydrogen and substituted or unsubstituted, alkyl and aryl. The alkylgroup may be straight or branched chain and preferably has from 1 to 8carbon atoms. The preferred aryl group is phenyl. The substituents forthe alkyl and aryl groups include alkyl and alkoxy having from 1 to 8carbon atoms, cyano and halogen. L³ may be selected from alkyl, aryl,halide, quinolates (e.g. 8-hydroxyguinoline), salicylaldehydes,salicylic acid, and maleonitriledithiolate (“mnt”). When A is S and Y isCN and L³ is “mnt” an emission between red and orange is expected.

The M (acetylacetonate)₃ complexes shown in FIG. 7 generate a blueemission. The metal ion M is selected from trivalent metals of Groups3-13 of the Periodic Table and the Lanthanides. The preferred metal ionsare Al⁺³, Ga⁺³ and In⁺³. The group R in FIG. 7 is the same as definedfor R in FIG. 3. For example, when R is methyl, and M is selected fromAl⁺³, Ga⁺³ and In⁺³, respectively, the wavelengths resulting from themeasurements of photoluminescence in the solid state is 415 nm, 445 nmand 457 nm, respectively (See J. Kido et al., “OrganicElectroluminescent Devices using Lanthanide Complexes”, Journal ofAlloys and Compounds, Vol. 92, pp. 30-33 (1993).

The metal bidentate complexes employed in the present inventiongenerally produce blue emissions.

Such complexes have the formula MDL⁴ ₂ wherein M is selected fromtrivalent metals of Groups 3-13 of the Periodic Table and theLanthanides. The preferred metal ions are Al⁺³, Ga⁺³, In⁺³ and Sc⁺³. Dis a bidentate ligand examples of which are shown in FIG. 8A. Morespecifically, the bidentate ligand D includes 2-picolylketones,2-quinaldylketones and 2-(o-phenoxy) pyridine ketones where the R groupsin FIG. 8A are as defined above.

The preferred groups for L⁴ include acetylacetonate; compounds of theformula OR³R wherein R³ is selected from Si, C and R is selected fromthe same groups as described above; 3,5-di(t-bu) phenol; 2,6-di(t-bu)phenol; 2,6-di(t-bia) cresol; and H₂Bpz₂, the latter compounds beingshown in FIGS. 8B-8E, respectively.

By way of example, the wavelength (λ_(cm)) resulting from measurement ofphotoluminescence in the solid state of aluminum (picolymethylketone)bis [2,6-di(t-bu) phenoxide] is 420 nm. The cresol derivative of theabove compound also measured 420 nm. Aluminum (picolylmethylketone) bis(OSiPh₃) and scandium (4-methoxy-picolylmethylketone) bis(acetylacetonate) each measured 433 nm, while aluminum[2-(O-phenoxy)pyridine] bis [2,6-di(t-bu) phenoxide] measured 450 nm.

Bisphosphonate compounds are another class of compounds which may beused in accordance with the present invention for the EL layers. Thebisphosphonates are represented by the general formula:

M² _(x)(O₃P-organic-PO₃)_(y)

M² is a metal ion. It is a tetravalent metal ion (e.g. Zr⁺⁴, Ti⁺⁴ andHf⁺⁴ when x and y both equal 1. When x is 3 and y is 2, the metal ion M²is in the divalent state and includes, for example, Zn⁺², Cu⁺² and Cd⁺².The term “organic” as used in the above formula means any aromatic orheterocyclic fluorescent compound that can be bifunctionalized withphosphonate groups.

The preferred bisphosphonate compounds include phenylene vinylenebisphonsphonates as for example those shown in FIGS. 9A and 9B.Specifically, FIG. 9A shows β-styrenyl stilbene bisphosphonates and FIG.9B shows 4,4′-biphenyl di(vinylphosphonates) where R is as describedpreviously and R⁴ is selected from substituted and unsubstituted alkylgroups, preferably having 1-8 carbon atoms, and aryl. The preferredalkyl groups are methyl and ethyl. The preferred aryl group is phenyl.The preferred substitutuents for the alkyl and aryl groups include atleast one substituent selected from aryl, halogen, cyano, alkoxy,preferably having from 1 to 8 carbon atoms.

The divalent metal maleonitriledithiolate (“mnt”) complexes have thestructural formula shown in FIG. 10. The divalent metal ion M³ includesall metal ions having a +2 charge, preferably transition metal ions suchas Pt⁺², Zn⁺² and Pd⁺². Y¹ is selected from cyano and substituted orunsubstituted phenyl. The preferred substituents for phenyl are selectedfrom alkyl, cyano, chloro and 1,2,2-tricyanovinyl.

L⁵ represents a group having no charge. Preferred groups for L⁵ includeP(OR)₃ and P(R)₃ where R is as described above or L⁵ may be a chelatingligand such as, for example, 2,2′-dipyridyl; phenanthroline;1,5-cyclooctadiene; or bis(diphenylphosphino)methane.

Illustrative examples of the emission wavelengths of variouscombinations of these compounds are shown in Table 1, as derived from C.E. Johnson et al., “Luminescent Iridium(I), Rhodium(I), and Platinum(II)Dithiolate Complexes”, Journal of the American Chemical Society, Vol.105, pg. 1795 (1983).

TABLE 1 Complex Wavelength* [Platinum(1, 5-cyclooctadiene) (mnt)] 560 nm[Platinum(P(OEt)₃)₂(mnt)] 566 nm [Platinum(P(OPh)₃)₂(mnt)] 605 nm[Platinum(bis(diphenylphosphino)methane) (mnt)] 610 nm[Platinum(PPh₃)₂(mnt)] 652 nm *wavelength resulting from measurement ofphotoluminescence in the solid state.

Molecular charge transfer complexes employed in the present inventionfor the EL layers are those including an electron acceptor structurecomplexed with an electron donor structure. FIGS. 11A-11E show a varietyof suitable electron acceptors which may form a charge transfer complexwith one of the electron donor structures shown in FIGS. 11F-11J. Thegroup R as shown in FIGS. 11A and 11H is the same as described above.

Films of these charge transfer materials are prepared by eitherevaporating donor and acceptor molecules from separate cells onto thesubstrate, or by evaporating the pre-made charge transfer complexdirectly. The emission wavelengths may range from red to blue, dependingupon which acceptor is coupled with which donor.

Polymers of aromatic and heterocyclic compounds which are fluorescent inthe solid state may be employed in the present invention for the ELLayers. Such polymers may be used to generate a variety of differentcolored emissions. Table II provides examples of suitable polymers andthe color of their associated emissions.

TABLE II POLYMER EMISSION COLOR poly(para-phenylenevinylene) blue togreen poly(dialkoxyphenylenevinylene) red/orange poly(thiophene) redpoly(phenylene) blue poly(phenylacetylene) yellow to redpoly(N-vinylcarbazole) blue

The rare earth mixed chelates for use in the present invention includeany lanthanide elements (e.g. La, Pr, Nd, Sm, Eu, and Tb) bonded to abidentate aromatic or heterocyclic ligand. The bidentate ligand servesto transport carriers (e.g. electrons) but does not absorb the emissionenergy. Thus, the bidentate ligands serve to transfer energy to themetal. Examples of the ligand in the rare earth mixed chelates includesalicyladehydes and derivatives thereof, salicylic acid, quinolates,Schiff base ligands, acetylacetonates, phenanthroline, bipyridine,quinoline and pyridine.

The hole transporting layers 20H, 21H and 22H may be comprised of aporphorinic compound. In addition, the hole transporting layers 20H, 21Hand 22H may have at least one hole transporting aromatic tertiary aminewhich is a compound containing at least one trivalent nitrogen atom thatis bonded only to carbon atoms, at least one of which is a member of anaromatic ring. For example, the aromatic tertiary amine can be anarylamine, such as a monoarylamine, diarylamine, triarylamine, or apolymeric arylamine. Other suitable aromatic tertiary amines, as well asall porphyrinic compounds, are disclosed in Tang et al., U.S. Pat. No.5,294,870, the teachings of which are incorporated herein in theirentirety by reference, provided any of such teachings are notinconsistent with any teaching herein.

The fabrication of a stacked organic LED tricolor pixel according to thepresent invention may be accomplished by either of two processes: ashadow masking process or a dry etching process. Both processes to bedescribed assume, for illustrative purposes, a double heterostructureLED construction, i.e., utilizing only one organic compound layer foreach active emission layer, with light emerging from the bottom glasssubstrate surface. It should be understood that multiple heterojunctionorganic LEDs having multiple organic compound layers for each activeemission layer, and/or inverted structures (with light emerging from thetop surface of the stack) can also be fabricated by one skilled in theart making slight modifications to the processes described.

The shadow masking process steps according to the present invention areillustrated in FIGS. 12(A-E). A glass substrate 50 to be coated with alayer of ITO 52 is first cleaned by immersing the substrate 50 for aboutfive minutes in boiling trichloroethylene or a similar chlorinatedhydrocarbon. This is followed by rinsing in acetone for about fiveminutes and then in methyl alcohol for approximately five minutes. Thesubstrate 50 is then blown dry with ultrahigh purity (UHP) nitrogen. Allof the cleaning solvents used are preferably. “electronic grade”. Afterthe cleaning procedure, the ITO layer 52 is formed on substrate 50 in avacuum using conventional sputtering or electron beam methods.

A blue emitting LED 55 (see FIG. 12B) is then fabricated on the ITOlayer 52 as follows. A shadow mask 73 is placed on predetermined outerportions of the ITO layer 52. The shadow mask 73 and other masks usedduring the shadow masking process should be introduced and removedbetween process steps without exposing the device to moisture, oxygenand other contaminants which would reduce the operational lifetime ofthe device. This may be accomplished by changing masks in an environmentflooded with nitrogen or an inert gas, or by placing the masks remotelyonto the device surface in the vacuum environment by remote handlingtechniques. Through the opening of mask 73, a 50-100 Å thick holetransporting layer (HTL) 54 and 50-200 Å thick blue emission layer (EL)56 (shown in FIG. 12B) are sequentially deposited without exposure toair, i.e., in a vacuum. An electron transporting layer (ETL) 58 having athickness preferably of 50-1000 Å is then deposited on EL 56. ETL 58 isthen topped with a semitransparent metal layer 60M which may preferablyconsist of a 10% Ag in 90% Mg layer, or other low work function metal ormetal alloy layer, for example. Layer 60M is very thin, preferably lessthan look. Layers 54, 56, 58 and 60M may be deposited by any one of anumber of conventional directional deposition techniques such as vaporphase deposition, ion beam deposition, electron beam deposition,sputtering and laser ablation.

An ITO contact layer 60I of about 1000-4000 Å thick is then formed onthe metal layer 60M by means of conventional sputtering or electron beammethods. For convenience herein, the sandwich layers 60M and 60I will bereferred to and shown as a single layer 60, which is substantially thesame as the layer 26 of FIG. 2. The low work function metal portion 60Mof each layer 60 directly contacts the ETL layer beneath it, while theITO layer 60I contacts the HTL layer immediately above it. Note that theentire device fabrication process is best accomplished by maintainingthe vacuum throughout without disturbing the vacuum between steps.

FIG. 12C shows a green emitting LED 65 which is fabricated on top oflayer 60 using substantially the same shadow masking and depositiontechniques as those used to fabricate blue emitting LED 55. LED 65comprises HTL 62, green emission layer 64 and ETL 66. A second thin(<100 Å thick, thin enough to be semi-transparent but not so thin tolose electrical continuity) metal layer 60M is deposited on ETL layer66, followed by another 1000-4000 Å thick ITO layer 60I to form a secondsandwich layer 60.

Shown in FIG. 12D is a red emitting LEE) 75 fabricated upon layer 60(upon 60I to be specific) using similar shadow masking and metaldeposition methods. Red emitting LED 75 consists of a HTL 70, a redemitting EL 72 and ETL 74. A top sandwich layer 60 of layers 60I and 60Mare then formed on LED 75. As described above for the embodiment of FIG.2, similarly, the top transparent ITO layer 60I can in an alternativeembodiment be replaced by an appropriate metal electrode serving also tofunction as a mirror for reflecting upwardly directed light back throughthe substrate 50, thereby decreasing light losses out of the top of thedevice. Each ETL layer 74, 66 and 58 has a thickness of 50-200 Å; eachHTL layer 54, 62 and 70 is 100-500 Å thick; and each EL layer 56, 64 and72 is 50-1000 Å thick. For optimum brightness and efficiency, each ofthe layers including the ITO/metal layers should be kept as close aspossible towards the lower end of the above ranges, but these ranges arenot meant to be limiting.

The formation of electrical contacts 51 and 59 on ITO layer 52, andelectrical contacts 88, 89, 92, 94 and 96 on the ITO portion 60I ofITO/metal layers 60 is then preferably accomplished in one step. Theseelectrical contacts may be indium, platinum, gold, silver orcombinations such as Ti/Pt/Au, Cr/Au or Mg/Ag. They may be deposited byvapor deposition or other suitable metal deposition techniques aftermasking off the rest of the device.

The final step in the shadow masking process is to overcoat the entiredevice with an insulating layer 97 as shown in FIG. 12E, with theexception of all the metal contacts 51, 59, 88, 89, 92, 94 and 96 whichare masked. Insulating layer 97 is impervious to moisture, oxygen andother contaminants thereby preventing contamination of the LEDs.Insulating layer 97 may be SiO₂, a silicon nitride such as Si₂N₃ orother insulator deposited by electron-beam, sputtering, or pyroliticallyenhanced or plasma enhanced CVD. The deposition technique used shouldnot elevate the device temperature above 120° C. inasmuch as these hightemperatures may degrade the LED characteristics. Note that the 120° C.is variable, and represents the softening point of typical organicmaterials used in the present invention.

The dry etching process for fabricating the LED stack according to theinvention is illustrated in FIGS. 13(A-F). Referring to FIG. 13A, aglass substrate 102 is first cleaned in the same manner as in theshadow-mask process described above. An ITO layer 101 is then depositedon the glass substrate 102 in a vacuum using conventional sputtering orelectron beam methods. An HTL 104, blue EL 105, ETL 106 and sandwichlayer comprising metal layer 107M and ITO layer 107I, all of generallythe same thicknesses as in the shadow-masking process, are thendeposited over the full surface of the ITO layer 101, using eitherconventional vacuum deposition, or in the case of polymers spin or spraycoating techniques. ITO/metal sandwich layer 107 consists of a less than100 Å thick, low work function metal layer 107M deposited directly onthe ETL layer 106, and a 1000-4000 Å thick ITO layer 107I on the metallayer 107M. On the entire top surface of ITO layer 107I, a 1000-2000 Åthick layer of silicon nitride or silicon dioxide masking material 108is deposited using low temperature plasma CVD. A positive photoresistlayer 109 such as HPR 1400 J is then spun-on the silicon nitride layer108. As shown in FIG. 13B the outer portions 110 (see FIG. 13A) of thephotoresist layer 109 are exposed and removed using standardphotolithographic processes. The exposed outer portions 110 correspondto the areas where the bottom ITO layer 101 is to be exposed andelectrically contacted. Referring to FIG. 13C, the outer regions 111(defined in FIG. 13B) of the silicon nitride layer 108 corresponding tothe removed photoresist areas, are removed using a CF₄:O₂ plasma. Then,using an ion milling technique or another plasma etch, the exposed outerportions of ITO/metal layers 107I and 107M are removed. An O₂ plasma isthen employed to sequentially remove the corresponding exposed outerportion of the ETL layer 106, EL layer 105, and HTL layer 104,respectively, and also to remove the remaining photoresist layer 109shown in FIG. 13D. Finally, a CF₄:O₂ plasma is again applied to removethe silicon nitride mask 108, with the resulting blue LED configurationshown in FIG. 13D.

The same sequence of dry etching process steps is used to fabricate agreen LED 115 atop the blue LED, except that SiNx 150 is overlaid asshown, followed by a photoresist mask 113 as shown in FIG. 13E to maskthe outer portion of ITO layer 101. Then the deposition of HTL layer114, green EL layer 116, and so on is performed (see FIG. 13F). The samephotolithography and etching techniques used for blue LED fabricationare then employed to complete the formation of the green LED 115. Thered LED 117 is then formed atop the green LED using substantially thesame dry etching process. A passivation layer 119 similar to layer 97 ofFIG. 12E is then deposited over the LED stack with suitable patterningto expose electrical contacts, as was described for the shadow maskingprocess. A photoresist mask is used to allow dry etching of holes inpassivation layer 119. Next, metal 152 is deposited in the holes. Afinal photoresist layer and excess metal is removed by a “lift-off”process.

Following the LED stack fabrication, whether performed by a shadow mask,dry-etching or other method, the stack must be properly packaged toachieve acceptable device performance and reliability. FIGS. 14(A-C)illustrate embodiments of the invention for facilitating packaging, andfor providing a hermetic package for up to four of the multicolor LEDdevices of the invention, for example. The same reference numerals usedin FIGS. 14(A-B) indicate the identical respective features as in FIG.12E. The package may also be used with the nearly identical structure ofFIG. 13F. Referring to FIG. 14A, after overcoating the entire devicewith an insulating layer 97, such as SiNx for example, access holes 120,122, and 124 are formed using known etching/photomasking techniques toexpose the topmost metal layers 60M′, 60M″, and 60MI′″, for the blue,green, and red LED (organic light emitting diode) devices, respectively,in this example. Thereafter, suitable metal circuit paths 126, 128, and130 (typically of gold material), are deposited in a path from theexposed metal layers 60M′, 60M″, and 60M′″, respectively, to edgelocated indium solder bumps 132, 133, and 134, respectively, usingconventional processing steps. Similarly, an anode electrode terminationis provided via the metal (Au, for example) circuit path 135 formed tohave an inner end contacting ITO layer 52, and an outer end terminatingat an edge located indium solder bump 136, all provided via conventionalprocessing. The device is then overcoated with additional insulatingmaterial such as SiNx to form an insulated covering with solder bumps132, 133, 134, and 136 being exposed along one edge. In this manner, theorganic LED device can be readily packaged using conventionaltechniques, or the packaging embodiment of the invention as describedimmediately below.

A method for making four multicolor LED devices on a common substrate Soin a packaged configuration will now be described, with reference toFIGS. 14A, 14B, and 14C, respectively, for another embodiment of theinvention. The starting material includes a glass substrate 50 coatedwith an overlayer of indium tin oxide (ITO) 52. The following steps areused to obtain the packaged multicolor organic LED array:

1. Mask ITO layer 52 to deposit an SiO₂ layer 138 in a concentric squareband ring pattern, in this example (some other pattern can be employed),on top of ITO layer 52 using conventional techniques.

2. Form four three-color LED stacks sharing common layers in region 140on the ITO layer 52 using methods as taught above for obtaining, forexample, either of the structures of FIGS. 12E or 13F, and 14A.

3. Deposit via shadow masking metal contacts 170 through 181; eachterminating at exterior ends on SiO₂ layer 138, for providing externalelectrical connecting or bonding pads 170′ through 181′, respectively.Note that contacts 126, 128, and 130 in FIG. 14A are the same as everysuccessive three of contacts 170-181, respectively. Each group of threecontacts, namely 170 through 172, 173 through 175, 176 through 178, and179 through 181, terminate at their interior or other ends to provide anelectrical connection with the metal layers 60M′, 60M″, 60M′″,respectively, of each of the four organic LED devices, respectively.Another metal contact 182 is deposited via shadow masking on an edge ofITO layer 52 common to all four of the LED devices, for providing acommon anode connection, in this example. Note that if throughappropriate masking and etching the four LED devices are made incompletely independent layers, four anode contacts, respectively, willhave to be provided for the latter array so that it can be operated in amultiplexed manner. The multicolor LED array being described in thisexample is a non-multiplexed array.

4. Deposit via shadow masking using an “L” shaped mask in two steps orvia photolithography, for example, a second SiO₂ layer 184 in acontinuous band or ring leaving exposed bonding pads 170′ through 181′,using either sputtering, or plasma enhanced CVD, or electron beamdeposition, for example.

5. Deposit Pb—Sn or other low temperature melting solder in a continuousband or ring 186 on top of the second SiO₂ layer or band 184.

6. Deposit on the bottom of a cover glass 188 a metal ring 190 to becoincident with the solder seal ring 186.

7. Place the assembly in an inert gas atmosphere, such as dry nitrogen,and apply heat to melt solder ring 186 to obtain an air tight seal, withthe inert gas trapped in interior region 192.

8. Install cover glass 188 over the assembly, as shown in FIG. 14B, withmetal ring 190 abutting against the solder ring 186.

Referring to FIG. 15, there is shown a display 194 which is an RGBorganic LED display. The dots 195 are ellipsis. A complete display as194 comprises a plurality of pixels such as 196. The pixels are arrangedas a XY matrix to cover the entire surface area of a glass sheet coatedwith ITO. Each pixel includes a stacked LED structure as that shown inFIG. 2. Instead of having fixed bias means as batteries 30, 31 and 32(FIG. 2) each of the lines of terminals designated in FIG. 2 as blue(B), green (G) and red (R) are brought out and coupled to suitablehorizontal and vertical scan processors 197 and 198, respectively, allunder control of a display generator 199 which may be a TV unit.Accordingly, each matrix of LEDs has at least two axes (x,y), and eachLED is at the intersection of at least two of the axes. Also, the x-axismay represent a horizontal axis, and the y-axis a vertical axis. It iswell known how to convert television signals such as the NTSC signalsinto the color components R, G and B for color displays. Monitors forcomputers which utilize red, green and blue for primary colors are alsowell known. The drive and control of such devices by vertical andhorizontal scanning techniques are also known. The entire array of pixelstructures deposited over the surface of the display is scannedemploying typical XY scanning techniques as using XY addressing. Thesetechniques are used in active matrix displays.

One can use pulse width modulation to selectively energize the red,green and blue inputs of each of the DH LED pixels according to desiredsignal content. In this manner, each of the LEDs in each line of thedisplay are selectively accessed and addressed and are biased by manymeans such as by pulse width modulation signals or by staircasegenerated voltages to enable these devices to emit single colors ormultiple colors, so that light emitted from said structures creates animage having a predetermined shape and color. Also, one can seriallyscan each of the xy axes, and serially energize selected ones of theLEDs in the matrix to emit light for producing an image with colorscreated serially vertically. Selected ones of the LEDs may besimultaneously energized.

As indicated above, the vertical layering technique shown in FIG. 2allows the fabrication of the three color DH LED pixel within extremelysmall areas. This allows one to provide high definition displays such asdisplays that have 300 to 600 lines per inch resolution or greater. Suchhigh resolution would be considerably more difficult to obtain usingprior art techniques in which the organic emission layers or fluorescentmedia generating the different colors are laterally spaced from oneanother.

Based on modern standards one can provide a LED device as shown in FIG.2 with an effective area small enough to enable hundreds of pixel diodesto be stacked vertically and horizontally within the area of a squareinch. Therefore, the fabrication techniques enables one to achieveextremely high resolution with high light intensity.

In FIG. 16, another embodiment of the invention is shown for amulticolor LED device including the stacking of up to N individual LEDs,where N is an integer number 1,2,3 . . . N. Depending upon the state ofthe technology at any future time, N will have a practical limit. Thestacked N levels of LEDs can, for example, be provided using either theshadow masking process steps previously described for FIGS. 12(A-E), orthe dry etching process illustrated in FIGS. 13A through 13F. The baseor bottom portion of the stacked array of FIG. 16 is a glass substrate102 as shown in FIG. 13F, for example, with an ITO layer 101 formed oversubstrate 102. The immediately overlying first LED device, and followingLED devices in this example, each include in succession over ITO layer101 an HTL layer 154, an EL layer 156, an ETL layer 158, a metal layer160, and an ITO layer 162. The No level LED device 164 further includesa topmost metal layer (see layer 152 of FIG. 13F) formed over theuppermost ITO layer 162 thereof. A passivation layer 119 is depositedover the stack, as in the color stack of FIG. 13F. The material for eachEL layer 156 of each LED device is selected for providing a particularcolor for the associated LED. As in the three color device, shorterwavelength (blue) devices must lie lower in the stack than the longerwavelength (red) devices to avoid optical absorption by the red emittinglayers. The color selected for each respective LED and the actual numberof stacked LEDs are dependent upon the particular application, and thedesired colors and shading capability to be provided. Such multi-colordevices can also be used in optical communications networks, where eachdifferent optical channel is transmitted using a different wavelengthemitted from a given device in the stack. The inherently concentricnature of the emitted light allows for coupling of several wavelengthsinto a single optical transmission fiber. In practical such stackedarrays, access holes are formed down to the ITO layer 162 of each devicefollowed by the deposition of appropriate metallization for facilitatingpackaging and electrical connection to each of the LED devices in thestack, in a manner similar to that described for the stacked multicolorLED device of FIGS. 14A, 14B, and 14C, for example.

This device can be used to provide a low cost, high resolution, highbrightness full color, flat panel display of any size. This widens thescope of this invention to displays as small as a few millimeters to thesize of a building, but to a practical limit. The images created on thedisplay could be text or illustrations in full color, in any resolutiondepending on the size of the individual LEDs.

The inventors recognized that if the multicolor organic LEDs, asdescribed above relative to the description of FIGS. 1A through 16, areimproved to be substantially fully transparent to a user when the OLEDdevices are de-energized, then such transparent organic light emittingdevices (hereinafter TOLED) are directly applicable for use in heads-updisplays, and other applications. For purposes of this description,heads-up displays are substantially transparent to a user whende-energized, thereby providing view through capability to the user.When a given one or more of the devices are energized, to emit light inthis example, the effected portion of the display would then illuminatean individual or multicolor display via the above-described organic LEDsincorporating the below-described embodiment of the invention. In orderto accomplish this, the present inventors conceived methods andapparatus for overcoming the difficulties involved in the prior art indepositing transparent electrical contacts on soft material such as usedin organic light emitting devices (OLEDS) without causing damage to theunderlying layers, as typically occurs when using known prior processes.The inventors recognize that in overcoming these problems, OLED devicescan then be used in heads-up displays that employ a variety of otherdisplay technologies, which themselves would benefit from thebelow-described embodiments of the invention for producing contacts thatare substantially transparent to a user. For example, applications thatmay benefit from the present invention are heads-up displays employed invisors on a motorcyclist helmet, or a jet fighter pilots helmet, eachpatterned with OLEDs for providing direct view displays for gauges,maps, itinerary, and so forth. Other applications employing such OLEDheads-up based displays may also include displays on windshields,teleprompters, and so forth. Video game displays and other type displaysmay also include OLED's of the present invention. Accordingly, theembodiments of the invention described below are believed to represent asignificant step forward in the state-of-art.

Methods are known for depositing transparent contacts on hard material,or materials that are unaffected by temperatures exceeding about 50° C.,such as silicon (Si) used in inorganic solarcells, for example. Itshould be noted that although the intended benefit of thebelow-described embodiment of the invention is for use in formingtransparent contacts on soft material such as organic layers, the methodand apparatus can also be used for depositing transparent contacts onhard materials.

It should be noted that the embodiments of the invention for providingOLED(s) as described above relative to the parent application U.S. Ser.No. 08/354,674, now U.S. Pat. No. 5,707,745 provide devices that have aluminescence band that is substantially red shifted from the absorptionband by as much as 0.5 eV (electron volt). As a result, the presentOLED(s) are highly transparent over their own emission spectrum andthroughout most of the visible spectrum, thereby providing a propertynot found in inorganic semiconductor light emitters. Through use of theembodiments of the invention as described below for providingsubstantially transparent contacts, the present inventors havediscovered a new class of vacuum-deposited organic luminescence deviceswhich are greater than 71% transparent when de-energized, and arecapable of emitting light from top and bottom diode surfaces with highefficiency when energized (approaching or exceeding 1% quantumefficiency).

To accomplish the above-described results, the first problem to beovercome by the inventors was to discover a metal capable of forming agood chemical bond with an underlying organic layer, to providemechanical stability. It was determined that one such metal can beprovided through the use of a metal alloy film of magnesium (Mg) andsilver (Ag). However, other metals and/or metal alloy films such asfluorine doped tin oxides, Ca, In, Al, Sm, Y, Yb, MgAl, and variousalloys containing these materials other than metal alloy films of Mg:Agmay be applicable for use (see Tang et al. U.S. Pat. No. 5,294,870).Films of Mg:Ag are presently considered to represent a preferredembodiment for the present invention. If the contact consists of asingle metal, the metal must have a low work function. When the contactconsists of a metal alloy, at least one of the metals must have a lowwork function. In using Mg:Ag, Mg has a low work function. Also, thechosen metal must insure a good electrical bond with the organic layer,as determined through experimentation with various materials. The goodelectrical bond ensures the metal contact or electrode will inject asufficient number of carriers into the organic layer.

After solving the first problem of establishing a metal or metal alloyfor providing both a good chemical bond and electrical contact with anunderlying organic layer, the next problem for the inventors was todetermine how to make the contact transparent while preserving theseother properties such as low electrical resistance. It was known that bymaking the metal layer very thin, a desired transparency for the layercould be obtained. However, the inventors recognized that the layer mustbe thick enough to protect the underlying organic layer from therequired next processing step, in this example, of depositing an indiumtin oxide (ITO) layer on top of the metal layer. Also, for example, athin Mg layer oxidizes quickly, and must be coated with ITO as soonafter being formed as possible to protect the Mg layer. Prior processesfor doing this are conducted at high temperature and high powerdeposition, which would damage the underlying organic layer.Accordingly, the inventors conceived a process for depositing the ITOlayer on the metal layer at very low power and room temperature,typically 22 C (72 F).

The ITO layer, in addition to being transparent, is also electricallyconductive, and therefore reduces the electrical resistance of themultilayer contact formed with the Mg:Ag. The ITO cannot be used byitself for it typically does not provide a good bond with organicmaterial (i.e. it does not stick well to organic material), andtypically it is not a good electron injector into the organicelectroluminescent material. The Mg:Ag layer, in this example, doesprovide a good bond to the organic layer and to the ITO, and is a goodelectron injector.

In FIG. 17, a cross-sectional view of an engineering prototype producedby the inventors for providing-a transparent organic light emittingdevice (TOLED) is shown. In this example, device 300 is grown on a glasssubstrate 302 pre-coated with a transparent indium tin oxide (ITO) thinfilm 304 with a sheet resistivity of typically 20 Ω (ohms)/square,depending on the thickness of the ITO film. Note that although substrate302 consists of transparent glass, in this example, it can also beprovided by any other transparent rigid support on which ITO can becoated, such as plastic material, for example. Also, the ITO film can bereplaced by any suitable electrical conducting oxide or a conductivetransparent polymer. Prior to deposition of the organic films, thesubstrate 302 was pre-cleaned using conventional techniques. Depositionwas performed by sublimation, in a vacuum of <10⁻⁶ Torr, of a 200 Åthick layer 306 of the hole conducting compoundN,N′-diphenyl-N,N′bis(3-methylphenyl)1-1′biphenyl-4,4′diamine (TPD),followed by a 400 Å thick layer 308 of the electron conducting andhighly electroluminescent Alq₃ (aluminum tris 8-hydroxyquinoline). A toplayer 310 providing an electron-injecting contact to the device 300 wasmade by deposition through a shadow mask (not shown) of a thin (50 Å-400Å) semi-transparent Mg—Ag alloy electrode (in an approximate atomicratio of 40Mg:1Ag), for example. Other atomic ratios, such as 50Mg:1Ag,can be used, depending upon the application, but note that the inventionis not meant to be limited to any particular ratio or contact metalcomposition. Finally, the TOLED device 300 is capped by a second 400 Åthick ITO layer 312, sputter-deposited onto the Mg—Ag surface of layer310. This second ITO layer 312 provides a continuous, transparentconducting surface on top of which a second TOLED can be grown (seeabove for description of FIGS. 12, 13, and 16). ITO layer 312 is made asthick as possible to reduce resistivity, while retaining acceptabletransparency. Electrical contacts 314 (negative polarity) and 316(positive polarity) are bonded to ITO layers 312 and 304, respectively,using conventional processing.

The output of the device 300 measured from both the top and substratedevice surfaces for a TOLED with a 100 Å thick Mg—Ag electrode 310, inthis example, is shown in FIG. 18. Typical operating conditions for suchTOLED devices 1 mm in diameter are 10⁻⁴A (ampere) and 10V (volts) drivevoltage (applied across terminals 314 and 316). The emission spectrafrom both surfaces are similar to conventional Alq₃-based devicesdescribed previously (C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett.,Vol. 51, 913 (1987); and P. E. Burrows and S. R. Forrest, Appl. Phys.Lett., Vol. 64, 2285 (1993)), although there is a slight (10 nm) shiftof the emission toward the red of the light emerging from the topcontact relative to the back contact. This may result from differencesin the absorption spectrum of the Mg—Ag as compared to the ITO films,and/or absorption due to interfacial states in the Mg—Ag/ITO electrode.The total internal quantum efficiency of light emission from this device300 example is 0.75%, which is comparable to the efficiency ofconventional Alq₃-based OLEDs. Approximately 10% higher intensity isemitted through the substrate 302 than from the top contact 312.

The transparency as a function of wavelength of the TOLED 300 with a 100Å thick film 310 is shown in detail in FIG. 19. At the short wavelengthedge of the plot, the device becomes non-transmissive due to acombination of ITO absorption, Mg—Ag absorption, and the strongmolecular transitions to the L_(a) and B_(b) states of Alq₃. Note,however, that the device is 63% transparent at the peak (530 nm)emission wavelength of Alq₃, and this transparency extends across thevisible red. Some loss is apparent in the infrared, also due toabsorption by the Mg—Ag top contact. FIG. 20 shows the transmission ofthe Mg—Ag contact 310 at a wavelength of 530 nm with Mg—Ag film 310thicknesses ranging from 50 Å to 400 Å. Transparencies of 92% wereobserved for the thinnest Mg—Ag films. The thinnest working TOLED deviceproduced thus far by the inventors has a Mg—Ag contact 310 thickness of75 Å, corresponding to a contact transparency of 81%, and a total devicetransparency of 71%. The slope of the straight line fit to the data ofFIG. 20 gives an optical absorption coefficient of the Mg—Ag of 1.1×10⁶cm⁻¹, which is consistent with a calculated skin depth of 180 Å.

Note that while the prototype TOLED devices described above emit in thegreen spectral region, the structure demonstrated should work equallywell for organic devices emitting in any region of the visible spectrum,since the large Franck-Condon red shift from absorption to emission ischaracteristic of many organic materials. Using simple fabricationprocesses, it is therefore possible to construct an independentlyaddressable stack of multi-color emissive elements. Furthermore, thisstructure should also be useful for polymer-based OLEDs, as previouslyindicated. For example, Mg:Ag alloy thin film layers with an overlyingITO layer are similarly described above for use as layers 26M and 26I ofthe OLED devices of FIGS. 2A, 2B and 2C, for providing improvedtransparent contacts 26; and for layers 60M and 60I of FIGS. 12B, 12C,12D, 12E, and layers 107M and 107I of FIGS. 13A, 13B, 13C, 13D, 13E, and13F, for providing improved transparent contacts 60, and 107,respectively. Similarly, Mg:Ag layers 310 with overlying ITO layers 312can be used in place of metal of metal layers 60M′, 60M″, and 60M′″ ofthe OLED device of FIG. 14A, and metal layers 160 of the OLED device ofFIG. 16. In this manner, viable TOLED multicolor devices are attainable.

The method and apparatus for placing transparent contacts onto organicmaterial layers, whether for organic light emitting devices, or otherdevices, will now be described. In this example, the first step is todeposit a thin film of Mg:Ag alloy in some preferred ratio, such as40:1, respectively, on top of a target organic layer, using thermalco-evaporation of constituent metal atoms from resistively heatedmolybdenum boats, in this example. As previously illustrated withreference to FIG. 20, the thickness of the metal alloy film is dominantin determining the transparency of the resultant contact. A system orapparatus for accomplishing this is shown schematically in FIG. 21. Notethat in the following description of the evaporation system, and ofother systems of the invention, the actual components used by theinventors in this example are given in a parts list presented below,along with a following manufactures index for providing the names andaddresses of the manufacturers supplying the components illustrated.

The evaporation system for depositing the Mg:Ag film on an organicsubstrate, as shown in FIG. 21, will now be described in detail.

Mg:Ag film evaporation is performed in a vacuum chamber (E1) with a basepressure of about 10⁻⁷ Torr, that is maintained by an Alcatel 150 1/secturbo pump (E2) in conjunction with Alcatel roughing pump (E3) and acold trap (E4). Ag and Mg source metals are loaded into molybdenum (Mo)boats (E5) which are resistively heated by 10 kW (E6) and 1 kW (E7)power supplies to evaporate or vaporize the Mg:Ag in this example. Thesubstrate (E8) is positioned 30 cm (d₁) above the Mo boats (E5) and isheld in place by a water-cooled non-rotating substrate holder (E9). Ashutter (E10), positioned in the path between the Mo boats (E5) and thesubstrate (E8), can be remotely and selectively operated to be in theopen or closed position to respectively enable or block Mg:Agevaporated-film deposition on the substrate (E8) by Mg:Ag vapor. Notethat in a preferred deposition system the single shutter (E10) isreplaced by two shutters (E10A, E10B, not shown) for independentlyblocking or enabling vapor flow of Mg and Ag, respectively, from anassociated boat (E5) to substrate (E8). The thickness of the depositedfilm is measured by a film thickness monitor (E11) located next to thesubstrate. Two more thickness monitors (E12, E13) are located one aboveeach of the Mo boats (E5), respectively, in order to provide independentmeasurements for the evaporation rates from the two boats.

The system of FIG. 21, as described, is operated using the followingsteps to deposit the Mg:Ag film of this example on a substrate (E8):

Position the-shutter (E10) in the closed position.

Pump the evaporation chamber (E1) until it reaches 1×10⁻⁶ Torr(preferred, but can range 10⁻³ Torr to 10⁻¹⁰ Torr).

Turn on the 10 kW power supply (E6) and increase its power output slowlyuntil Ag starts to melt.

Set Ag density and acoustic impedance parameters on the substrate-filmthickness monitor (E11).

Set the output of the 10 kW power supply (E6) so that deposition rate ofAg, as registered by the substrate thickness monitor (E11), is 0.1 Å/s,in this example, but can be as high as 5 Å/s. Note the rate (R1)registered by Ag thickness monitor (E12).

Maintain R1 constant throughout the deposition process by adjusting theoutput of 10 kW power supply (E6), as required.

Set Mg parameters on the substrate-film thickness monitor (E11).

Set the output of the 1 kW power supply (E7) so that deposition rate ofMg, as registered by the substrate thickness monitor (E11), is 5 Å/s,which in this example is preferred but can otherwise range 0.1 Å/s to 10Å/s. Note the rate (R2) registered by Mg thickness monitor (E13).

Maintain R2 constant throughout the deposition process, by adjusting theoutput of 1 kw power supply (E7), as required.

Mount the substrate (E8) onto the substrate holder (E9).

Position the shutter (E10) in the open position.

Deposit ˜100 Å of Mg:Ag alloy, which is preferred in this example, butcan otherwise range 50 Å to 500 Å.

After the Mg:Ag alloy film is deposited on substrate (E8) the substrateis transferred from the thermal evaporation chamber (E1), via a loadlock chamber (S4), (see FIG. 22), to a sputtering chamber (S1) of an RFsputtering system. During this transfer, the sample is kept in vacuum,or inert atmosphere such as nitrogen or argon gas atmosphere, throughuse of the load lock chamber (S4), in this example.

Next, an indium-tin oxide (ITO) film is deposited on top of the Mg:Agmetal alloy film by RF sputtering in chamber (S1). Resistivity of thisITO film is 1.5×10⁻³ Ωcm. In this example, the ITO film is 400 Å thick.

The sputtering system, as shown in FIG. 23, in this example, is housedin a high vacuum chamber (S1) with a base pressure ranging between1×10⁻³ Torr to 1×10⁻¹⁰ Torr (lower base pressure is preferred to reduceinteraction with ambient gases), that is maintained by CTI-CryogenicsCryo-Torr 8 cryo pump (S2). Attached to this chamber via gate valve (S3)is a load-lock chamber (S4) with base pressure ranging from atmosphericto 1×10⁻¹⁰ Torr (lower base pressure is preferred), that is maintainedby Leybold Turbovac 50 turbo pump (S5). Through the load-lock chamber(S4) samples are introduced into the sputtering system and extracted outat completion of deposition. In this example, the sputtering target(S6), manufactured by Pure Tech., Inc. is 10% SnO₂ and 90% In₂O₃ byweight, with 99% purity. Target (S6) has a diameter of two inches with athickness of one-eighth of an inch. A copper backing plate (S17),one-eighth inch thick, is attached by vacuum epoxy to the back of thetarget (S6) to prevent target (S6) from overheating and cracking. Thetarget (S6) is housed in an AJA International magnetron sputtering gun(S7), powered by advanced Energy RF Power supply (S8) (with 600 Wmaximum RF power output, and operating frequency of 13.56 MHz) inconjunction with Advanced Energy auto-tunable impedance matching network(S9). Any RF power supply that provides at least 20 W RF power issufficient. A shutter (S10), positioned over the sputtering gun (S7) andthe target (S6), can be selectively positioned in the open or closedposition to respectively enable or prevent sputtered-film deposition onthe substrate (S11). The substrate (S11) is positioned 15 cm(d₂) abovethe shuttered target (S6) and is held in place by a water-coolednon-rotating substrate holder (S12). Note that d₂ can preferably rangefrom 5 cm to 30 cm, but can be greater. The thickness of the depositedfilm is measured by a calibrated film thickness monitor (S13) locatednext to the substrate (S11). The sputtering gas is a mix of argon (Ar)(99.9999% pure) and oxygen (O₂) (99.998% pure). The flow of gasses intothe sputtering chamber (S1) is controlled by individual MKS mass flowcontrollers (S14). The gas pressure inside the sputtering chamber (S1)is monitored by an MKS Baratron type 121A absolute pressure transducer(S15) and controlled by a butterfly valve (S16) positioned in front ofthe cryo pump (S2).

The sputtering system of FIG. 23 is operated to deposit an ITO layer ontop of the Mg:Ag film using the following operational steps:

Remove the sample from vacuum chamber (E1).

Introduce the sample into the load lock chamber (S4).

Pump down the load lock chamber until it reaches its base pressure.

Transfer the sample into the sputtering chamber (S1) and position itover the sputtering target (S6).

Position the target shutter (S10) in the closed position.

Set argon (Ar) gas flow into the sputtering chamber at a preferred rateof 200 sccm (can range 20 sccm to 1,000 sccm depending upon the systempumping speed), and oxygen (O₂) flow preferably at 0.1 sccm (can range0.0 sccm to 100 scam dependent upon both the Argon gas flow rate and thesputtering power, where greater flow of O₂ is required for highersputtering power).

Set the butterfly valve (S16) to maintain the chamber pressure of 20mTorr, preferred in this example, or as low as 1 mTorr, but must besufficient to allow for ignition of the plasma, and to sustain theplasma.

Set the RF power supply (S8) to an output power between 15 W to 30 W andenable it. When the auto-tuned impedance matching network (S9) finds theoptimal setting the plasma should ignite.

Reduce the pressure inside the sputtering chamber (S1) to 5 mTorr bysetting the butterfly valve (S16), in order to sustain the plasma as thepower is decreased in the next step.

Slowly reduce the output power on the RF power supply (S8) equal to orless than 5 W, making sure impedance matching network (S9) has enoughtime to respond to the change.

Position the target shutter (S10) in the open position.

Deposit between 50 Å to 600 Å of ITO, dependent upon the desired balancebetween transparency of the ITO layer, and its electrical conductivity.

Note that an important embodiment of the invention is the use of low RFpower (5 W or less) in sputtering ITO to deposit a thin film of ITO ontothe Mg:Ag layer in this example, to avoid damaging the underlyingorganic layer. The RF power can be reduced to below 5 W by usingdifferent gas mixtures in the sputtering chamber, such as 1 Xe:10Ar, or1 CH₃:20Ar, to further avoid damaging the organic layer whilemaintaining the growth rate for the ITO film. It is preferred that theRF power in the sputtering chamber be slowly reduced to the minimumwattage for sustaining ignition of the plasma.

A parts list, as given below in Table 1, for reference designations (E1)through (E13), for the deposition system of FIG. 21, and (S1) through(S17), for the sputtering system of FIG. 23, lists the description, partor model number, and associated manufacturer. Table 1 is followed by amanufacturer's index listing, providing the identified manufacturer'snames in full along with their last known address and telephone numbers.It should be noted that the invention is not meant to be limited to useof the illustrated components and parts, and manufacturers, and isprovided herein only to fully illustrate apparatus employed by theinventors in developing their invention through experimental engineeringprototypes.

TABLE 1 ITEM # ITEM DESCRIPTION MODEL # MANUF. E1 Vacuum Chamber DV-502ADenton E2 150 1/s Turbo Pump CFF-450 Alcatel Turbo E3 MechanicalRoughing 20008A Alcatel Pump E4 Liquid Nitrogen Cold standard DentonTrap E5 Molybdenum Source standard Mathis Boats E6 10 kW Power Supplystandard Denton E7 1 kW Power Supply standard Denton E8 User Provided —— Substrate E9 Water Cooled custom Denton Substrate Holder E10 Shutterstandard Denton E11 Film Thickness STM-100/MF Sycon Monitor E12 FilmThickness STM-100/MF Sycon Monitor E13 Film Thickness STM-100/MF SyconMonitor S1 Vacuum Chamber — DCA Inst. S2 Cryogenic Pump Cryo-Torr 8 CTIS3 Gate Valve (6 inch GC-4000M MDC diameter) S4 Load Lock Chamber customMDC (6 in., 6 way cross) S5 Turbo Pump Turbovac 50 Leybold S6 10% SnO₂,90% In₂O₃ with Cu Pure Tech target (2 in. diam.) backplate (S17) S7 RFMagnetron — AJA Sputtering Gun Intern. S8 600 W 13.56 MHz RF RFX-600Adv. Power Supply Energy S9 Impedance Matching ATX-600 Adv. NetworkEnergy S10 Integral part of S7 — AJA Intern. S11 User Provided custom —Substrate S12 Water Cooled custom DCA Inst. Substrate Holder S13 FilmThickness STM-100/MF Sycon Monitor S14 Mass Flow Controllers 1259C MKSS15 Absolute Pressure Baratron MKS Transducer #121A S16 Butterfly ValveL6691-301 Varian S17 Cu Backplate Epoxied to Pure Tech (S6)

Manufacturer Index:

Adv. Energy:

Advanced Energy Industries, Inc.

1600 Prospect Parkway, Fort Collins, Colo. 80525

(303) 221-4670

AJA Inern.:

AJA International

North Scituate, Mass. 02060

(800) 767-3698

Alcatel:

Alcatel Vacuum Products, Inc.

Hingham, Mass. 02043

(617) 331-4200

CTI:

CTI-Cryogenics

Mansfield, Mass. 02048

(508) 337-5000

DCA Inst.

DCA Instruments, Inc.

400 West Cummings Park, Suite 3900, Woburn, Mass. 01801

(617) 937-6550

Denton:

Denton Vacuum, Inc.

Moorestown, N.J. 08057

(609) 439-9100.

Leybold:

Leybold Vacuum Products, Inc.

Export, Pa. 15632

(800) 443-4021

Mathis:

R.D. Mathis Co.

2840 Gundry Ave., P.O. Box 6187, Long Beach, Calif. 90806

310) 426-7049

MDC:

MDC Vacuum Products Corp.

Hayward, Calif. 94545

(510) 887-6100

MKS:

MKS

6 Shattuck Rd., Andover, Mass. 01810

(508) 975-2350

Pure Tech:

Pure Tech, Inc.

Carmel, N.Y. 10512

(914) 878-4499

Sycon:

Sycon Instruments

6757 Kinne St., East Syracuse, N.Y. 13057

(315) 463-5297

Varian:

Varian Vacuum Products

Lexington, Mass. 02173

(800) 8-VAAIAN.

In another embodiment of the invention for growing the ITO layer 312onto the Mg:Ag film 310, in this example (see FIG. 17), it was recentlydiscovered that the ITO growth rate can be increased by growing thefirst 50-100 Å by the non-destructive (slow) method given above,followed by stepping up the rate to make the contact 312 thicker(typically 400-1000 Å) using higher power settings for RF power supply(S8) (20 W-40 W, for example). Since there is already a protective capof 50-100 Å grown at lower power settings for (S8) (1 W-7 W, forexample), this second, quickly grown ITO layer does not have the abilityto penetrate the first ITO slow-grown contact and destroy the underlyingMg:Ag and organic layers 310, 308, respectively. It was also discoveredthat Alq₃ and related compounds for the organic layers are very immuneto this damage, whereas the blue compounds are vulnerable. As a resultthe inventors now put in a “double heterostructure” (see FIG. 1A), e.g.first the TPD 306, then a blue emitter material layer ranging inthickness between 50 Å to 1,000 Å, followed by a layer of Alq₃ rangingin thickness from 300 Å to 1,000 Å. The resultant TOLED device stillluminesces blue.

Those with skill in the art may recognize various modifications to theembodiments of the invention described and illustrated herein. Suchmodifications are meant to be covered by the spirit and scope of theappended claims. For example, a multicolor stacked LED device, such asthe above-described three color device of FIG. 2, in another embodimentof the invention can be provided by forming LED 20 from a polymer deviceas shown in FIG. 1C, or from a deposited metal phosphonate film, ratherthan having all three layers laid down in vacuo. The two remainingstacked LEDs could be formed by vapor deposition, or other techniques.Also, the Mg:Ag alloy can range from 1Mg:1Ag to 40Mg:1Ag to 100% Mg withproper chemistry. In yet another embodiment the ITO layers for the TOLEDdevice can be formed by presputtering, spraying, or dipping.

What is claimed is:
 1. A method of fabricating a multicolor lightemitting device (LED) structure that is substantially transparent whende-energized, comprising the steps of: forming a first transparentconductive layer upon a transparent substrate; depositing asubstantially transparent first hole transporting layer upon said firsttransparent conductive layer; depositing a substantially transparentfirst organic emission layer upon said first hole transporting layer toprovide a first emission color; depositing via vapor deposition asubstantially transparent first electron transporting layer upon saidfirst organic emission layer; depositing via sputtering a secondtransparent conductive layer upon said first electron transportinglayer, said second transparent conductive layer adapted to receive afirst bias potential; depositing a substantially transparent second holetransporting layer upon said second transparent conductive layer;depositing a substantially transparent second organic emission layerupon said second hole transporting layer to provide a second emissioncolor; depositing via vapor deposition a substantially transparentsecond electron transporting layer upon said second organic emissionlayer; and depositing via sputtering a third transparent conductivelayer upon said second electron transporting layer, said thirdtransparent conductive layer adapted to receive a second bias potential.2. The method of claim 1, further including the step of shadow masking aregion of said first transparent conductive layer prior to depositingsaid first hole transporting layer to expose said region of said firsttransparent conductive layer thereby enabling said first bias potentialto be applied between said second transparent conductive layer and saidregion of said first transparent conductive layer.
 3. The method ofclaim 1, further including the step of etching away a region of saidfirst hole transporting layer to expose a portion of said firsttransparent conductive layer thereby enabling said first bias potentialto be applied between said second transparent conductive layer and saidexposed portion of said first transparent conductive layer.
 4. Themethod of claim 1, wherein said steps of vapor deposition for depositingthe substantially transparent first electron transporting layer uponsaid first organic emission layer, and the substantially transparentsecond electron transporting layer upon said second organic emissionlayer, each include: placing said substrate into a vacuum chamber withone of the first and second organic emission layers to be coated exposedon said substrate; selecting a metal or metal alloy for placement insaid chamber to be deposited on one of said first and second organicemission layers as an electron transporting layer; reducing the pressurein said vacuum chamber to about 1×10⁻⁶ Torr; melting the metal or metalalloy to produce a desired deposition rate for vapor depositing themetal or metal alloy on the exposed one of said first and second organiclayers; monitoring the thickness of said metal or metal alloy beingdeposited; and blocking the deposition of the vaporized metal or metalalloy to the one of said first and second organic emission layers when adesired thickness is reached.
 5. The method of claim 4, wherein saidsteps of depositing via sputtering a second transparent conductive layerupon said first electron transporting layer, and a third transparentconductive layer upon said second electron transporting layer, eachinclude the steps of: placing said substrate into a load lock chamberwith respectively one of the first and second electron transportinglayers to respectively receive a second or third conductive layer,exposed; reducing the pressure in said load lock chamber to 1×10⁻⁷ Torr;transferring the substrate under vacuum from said load lock chamber intoa sputtering chamber; positioning said substrate over a sputteringtarget; establishing a flowrate of argon gas into said sputteringchamber; establishing a flowrate of oxygen into said sputtering chamber;maintaining the pressure in said sputtering chamber at 20 mTorr;establishing a level of RF power and impedance matching for igniting aplasma to begin the sputtering of material from said target to anexposed one of said first and second electron transporting layer, byplacing a target shutter in an open position; slowly reducing the levelof RF power to a minimum level for sustaining ignition of said plasma;and closing said target shutter after sputter depositing a requiredthickness of the second or third transparent conductive layer.
 6. Themethod of claim 5, wherein the flowrate of argon gas is established atabout 200 sccm.
 7. The method of claim 4, wherein said first and secondelectron transporting layers are each formed from a metal alloy ofMg:Ag.
 8. The method of claim 7, wherein said melting step includes thesteps of: establishing a deposition rate for the Ag at 0.1 Å/s; andestablishing a deposition rate for the Mg at 5 Å/s.
 9. The method ofclaim 5, wherein the flowrate of oxygen is established at 0.1 sccm.